using Org.BouncyCastle.Math;
using Org.BouncyCastle.Math.EC;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Parameters;

namespace Org.BouncyCastle.Crypto.Agreement
{
	/**
	 * P1363 7.2.1 ECSVDP-DH
	 *
	 * ECSVDP-DH is Elliptic Curve Secret Value Derivation Primitive,
	 * Diffie-Hellman version. It is based on the work of [DH76], [Mil86],
	 * and [Kob87]. This primitive derives a shared secret value from one
	 * party's private key and another party's public key, where both have
	 * the same set of EC domain parameters. If two parties correctly
	 * execute this primitive, they will produce the same output. This
	 * primitive can be invoked by a scheme to derive a shared secret key;
	 * specifically, it may be used with the schemes ECKAS-DH1 and
	 * DL/ECKAS-DH2. It assumes that the input keys are valid (see also
	 * Section 7.2.2).
	 */
	public class ECDHBasicAgreement
		: IBasicAgreement
	{
		protected internal ECPrivateKeyParameters privKey;

		public void Init(
			ICipherParameters parameters)
		{
			if (parameters is ParametersWithRandom)
			{
				parameters = ((ParametersWithRandom)parameters).Parameters;
			}

			this.privKey = (ECPrivateKeyParameters)parameters;
		}

		public virtual BigInteger CalculateAgreement(
			ICipherParameters pubKey)
		{
			ECPublicKeyParameters pub = (ECPublicKeyParameters) pubKey;
			ECPoint P = pub.Q.Multiply(privKey.D);

			// if ( p.IsInfinity ) throw new Exception("d*Q == infinity");

			return P.X.ToBigInteger();
		}
	}

}
